Soft and elastic nonwoven fabrics of polypropylene and its copolymers with α-olefins such as ethylene which lead to substantially softer and more extensible constructions are desirable, yet often difficult to obtain with the right balance of properties. U.S. Pat. Nos. 3,853,969 and 3,378,606, suggest the formation of in situ blends of isotactic polypropylene and “stereo block” copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene to yield blends which may be fabricated to obtain soft and elastic nonwoven fabrics. Similar results are discussed in U.S. Pat. Nos. 3,262,992, 3,882,197, and 3,888,949, which suggests the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an α-olefin, containing between 6-20 carbon atoms, which are softer and have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and α-olefin are described wherein the α-olefin is hexene, octene or dodecene.
Examples of propylene homopolymers containing different levels of isotacticity in different portions of the molecule are described in U.S. Pat. No. 5,594,080, in the article in 117 JOURNAL AMERICAN CHEMICAL SOCIETY 11586 (1995), in the article in 119 JOURNAL AMERICAN CHEMICAL SOCIETY 3635 (1997), in the journal article in 113 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 8569-8570 (1991), and in the journal article in 28 JOURNAL MACROMOLECULES 3771-3778 (1995). U.S. Pat. Nos. 5,723,217; 5,726,103; 5,736,465; 5,763,080; and 6,010,588 suggest several metallocene catalyzed processes to make polypropylene suitable for fibers and fabrics. U.S. Pat. No. 5,891,814 discloses a dual metallocene-generated propylene polymer used to make spunbond fibers. WO 99/19547 discloses a method for producing spunbonded fibers and fabric derived from a blend of a propylene homopolymer and a copolymer of polypropylene. U.S. Pat. No. 6,342,565, U.S. 2005/0130544 A1 and U.S. 2006/0172647 discloses a fiber or nonwoven fabric.
These past disclosures have generally taught that fabricating a nonwoven fabric from a fiber to form a soft or extensible article generally requires the use of a semicrystalline polymer. These semicrystalline polymers are most conveniently made at a high molecular weight (above about 250,000 daltons weight average molecular weight), yet the process of manufacture of the fiber and the fabric, especially meltblown fibers and fabrics, requires a polymer of a lower molecular weight (less than about 250,000 daltons weight average molecular weight). The lower molecular weight is typically achieved by free radical assisted thermal processes (“controlled rheology”). While isotactic polypropylene has been produced at a low enough molecular weight for it to be used without post-reactor degradation, less crystalline polymers containing limited amounts of crystallinity have not been made useful for the fabrication of a fiber and a nonwoven fabric.